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(#) subscribeOn called with the main thread scheduler

!!! ERROR: subscribeOn called with the main thread scheduler
   This is an error.

Id
:   `SubscribeOnMain`
Summary
:   subscribeOn called with the main thread scheduler
Severity
:   Error
Category
:   Correctness
Platform
:   Any
Vendor
:   slack
Identifier
:   slack-lint
Contact
:   https://github.com/slackhq/slack-lints
Feedback
:   https://github.com/slackhq/slack-lints
Min
:   Lint 8.7+
Compiled
:   Lint 8.7+
Artifact
:   [com.slack.lint:slack-lint-checks](com_slack_lint_slack-lint-checks.md.html)
Since
:   0.1.0
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/main/java/slack/lint/rx/RxSubscribeOnMainDetector.kt)
Tests
:   [Source Code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/test/java/slack/lint/rx/RxSubscribeOnMainDetectorTest.kt)
Copyright Year
:   2021

Calling `subscribeOn(AndroidSchedulers.mainThread())` will cause the
code ran at subscription time to be executed on the main thread - that
is, code above this line.
Typically this is not actually desired, and instead you want to use
observeOn(AndroidSchedulers.mainThread()) which will cause the code
below this line to be run on the main thread (eg the code inside your
subscribe() block).

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/com/slack/lint/Foo.java:8:Error: This will make the code for the
initial subscription (above this line) run on the main thread. You
probably want observeOn(AndroidSchedulers.mainThread()).
[SubscribeOnMain]
    obs.subscribeOn(AndroidSchedulers.mainThread());
        -------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are the relevant source files:

`src/io/reactivex/rxjava3/android/schedulers/AndroidSchedulers.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package io.reactivex.rxjava3.android.schedulers;

import io.reactivex.rxjava3.core.Scheduler;

public final class AndroidSchedulers {
    public static Scheduler mainThread() {
        return null;
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`src/com/slack/lint/Foo.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package com.slack.lint;

import io.reactivex.rxjava3.core.Observable;
import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers;

public class Foo {
  public void bar(Observable obs) {
    obs.subscribeOn(AndroidSchedulers.mainThread());
  }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://github.com/slackhq/slack-lints/tree/main/slack-lint-checks/src/test/java/slack/lint/rx/RxSubscribeOnMainDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `RxSubscribeOnMainDetector.subscribeOnMain_fullyQualified_fails_java`.
To report a problem with this extracted sample, visit
https://github.com/slackhq/slack-lints.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project. This lint check is included in the lint documentation,
   but the Android team may or may not agree with its recommendations.

```
// build.gradle.kts
lintChecks("com.slack.lint:slack-lint-checks:0.8.2")

// build.gradle
lintChecks 'com.slack.lint:slack-lint-checks:0.8.2'

// build.gradle.kts with version catalogs:
lintChecks(libs.slack.lint.checks)

# libs.versions.toml
[versions]
slack-lint-checks = "0.8.2"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
slack-lint-checks = {
    module = "com.slack.lint:slack-lint-checks",
    version.ref = "slack-lint-checks"
}
```

0.8.2 is the version this documentation was generated from;
there may be newer versions available.

[Additional details about com.slack.lint:slack-lint-checks](com_slack_lint_slack-lint-checks.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("SubscribeOnMain")
  fun method() {
     subscribeOn(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("SubscribeOnMain")
  void method() {
     subscribeOn(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection SubscribeOnMain
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="SubscribeOnMain" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'SubscribeOnMain'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore SubscribeOnMain ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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